A newly designed opposed free piston internal combustion hydraulic power generator is proposed in this paper. The pressure of the hydraulic power output of the proposed hydraulic power generator is substantially constant and the hydraulic flow rate is variable. The frequency of free piston changes from zero to maximum in response to the hydraulic flow rate and each gas cycle is always performed under an optimum working condition independent of the power output. If the proposed hydraulic power generator is combined with a variable displacement hydraulic motor, it will be possible to obtain excellent torque-speed characteristic, easy operation, and superior fuel economy. A fundamental test apparatus was constructed and tested in this work. The test apparatus uses an opposed free piston, two stroke cycle, direct fuel injection, and compression ignition engine.